Description of the Rotascope. — 265 
Arr. III.—Description of an Apparatus called the Rotascope; for 
exhibiting several phenomena and illustrating certain laws of ro= 
tary motion; by Watrer R. Jounson, Professor of Mechanics 
and Natural Philosophy in the Franklin Institute, Philadelphia. 
Taz laws relating to rotary motion, have been frequently made 
the subjects of profound analysis, and the transactions of some.learn- 
ed societies of Europe contain numerous papers, expressly designed 
to elucidate this branch of mechanical science. 
Nearly all the discussions of the subject, however, which have 
hitherto appeared, have been of the abstract and transcendental 
character. Hence they have, in few instances, either interested or 
instructed the cultivators of practical science, and those who have 
long been familiar with mechanical forees and motions are frequently 
quite unacquainted, except by casual occurrences, with any of the 
curious laws which concern the positions and changes of position in: 
€ axes of rotation, in revolving bodies. Our common books of 
mechanics generally contain concise accounts of the doctrines of r0- 
lary motion, limited for the most part, however, to the consideration 
of central forces, the centre of percussion and of gyration, and the 
centre of spontaneous rotation, to which may be added that of oscil- 
latory motion. 
The forces tending to change the position of the axes of rotation 
tte generally either wholly omitted, or if concisely stated in an ab- 
“Tact form, are apparently regarded as incapable of experimental 
illustration. The whirling table of Mr. Ferguson is an ingemous ap- 
patatus for exhibiting the amount and directions of the several forces 
*xetted by a body in its own fixed plane of revolution. But that in- 
ent makes no provision for the phenomena above referred to. 
m we consider that the extensive diffusion of a branch of 
knowledge often depends on the facility with which its elements can 
“made apparent to the understanding, we are at no loss in estima~ 
tng the practical value of philosophical mstraments, whether intend- 
ed for demonstration, or for research. Of this truth the machine of 
Ood may be taken as an illustration. This machine gives a most 
elegant and satisfactory exhibition of the principles of uniform, accel- 
“ated and retarded motions, as dependent on the force of gravity. 
5 the motions in the machine may be so slow as to reduce the re 
“stance of the air to an unimportant element, and the friction and ine 
on. XXI.—No, 2, 34 
